Dunlop GB 1195241 patent relates to the slope of leading (butress) face of a farm tractor tire traction bar having a greater angle (slope) with respect to the radial direction with respect to the tire than the same angle for the trailing face of the traction bar. The load applied to the leading edge at contact with the surface is spread to bear over a larger area reducing the distortion and deflection of the casing and traction bar. The trailing edge being less supported than the leading edge results in less bar movement over the ground as it moves out of contact and, therefore, less wear. Both the leading and trailing faces are arranged to blend in a smooth curve with the rubber at the base of the tread by utilizing a relatively large radius of curvature. In an improvement patent GB 1374992 by Dunlop, the relative inclination of the leading face with respect to the trailing face was changed so that they were approximately equal at the shoulder region. This improvement was made to increase the resistance of the tread bar to shoulder wear caused by the concentration of lateral cornering forces in this region. Once again, a relatively large radius (25 mm to 30 mm) was used along the whole length of the tread bar to blend in a smooth curve with the rubber at the base of the bar.
In comparison with the two patents above, Pommier in his patent U.S. Pat. No. 4186788 shows traction bars adjacent the circumferential plane of symmetry having a leading edge with an abrupt slope (smaller angle with respect to the radial direction) and a gentle slope (larger angle) for the trailing edge. In contrast, at the shoulder region it reverses to a gentle slope on the leading edge and an abrupt slope on the trailing edge. Angles of 0.sup.o to 8.sup.o with respect to the radial direction were used for abrupt slopes and 6.sup.o to 20.sup.o were used for gentle slopes. Improved adherence properties were quoted, which are preserved as wear progresses. The leading and trailing faces show an abrupt change (small radius) at the base of the tread in this patent by Pommier (FIG. 2 of U.S. Pat. No. 4186788).
For the drive axle traction tires of heavy duty trucks, the contact surface ratio must be adequate to support large vertical loads. Therefore, truck tire traction bars become tread elements closely spaced together in a circumferential direction. Also, for convenience in using a truck tire at any wheel position, with either forward or backward rotation, these tires are classically non-directional tread designs. In contract, the farm tractor tires referenced above are unidirectional designs with their forward rotation specified. When tread elements are closely spaced for a proper contact surface ratio there is not sufficient groove width for the lateral grooves to have (1) the desired leading face and trailing face slopes with respect to the radial direction, (2) the adequate groove depth for proper traction throughout the tire's life and (3) a smooth transition at the base of the tread element without a compromise of tread design parameters.
The patent U.S. Pat. No. 4284115 by Ohnishi addressed the compromise in tread design for a bias heavy duty tire by setting the slope (slant angle) of the groove walls and the curvature of the grooves between walls (leading and trailing faces) and the bottom thereof within a specified range. The groove width being reduced at the bottom of the grooves substantially increases the probability of cracks. Ohnishi shows a larger transition radius between the groove bottom and the leading face than between the groove bottom and the trailing face to reduce cracking. Ohnishi also shows the slope of the leading face of the lateral groove having an angle (slope) with respect to the radial different than that of the trailing face. However, for this bias non-directional tread design the leading face has a larger angle than the trailing face on one side of the centerline portion of the tread and a smaller angle than the trailing face on the other side. Decreasing angles from the shoulder to the centerline are also disclosed for a smooth transition from one side to the other. A combination of the variable radius groove bottom contour and the difference in the slope of the leading and trailing faces in one tread design was not disclosed in this patent. Also, test results were given for large off-road or earthmover tires only.
This background information does not teach one skilled in the art how to design the tread of a drive axle tire. Conflicting solutions were obtained to solve different problems.